The invention relates to a device for powdering printed sheets of paper or similar material by means of a powdering device, which is located inside a sheet delivery mechanism, where a charging device is assigned to the sheet for electrostatic charging.
In DE 44 27 904 A1 powdering equipment is disclosed with which powder is electrostatically charged before it is blown onto a printed paper sheet. Electrostatically charged powder certainly adheres better to paper sheets, but its adhesive strength needed to be improved. In DE 29 36 754 A1 a device is disclosed for the electrostatic application of material particles transported in a stream of gas onto a paper sheet. In this process, an electrical field is generated in the vicinity of the sheet of paper, into which the material particles, in particular, powder, is blown. As a result of the field lines, the powder is guided onto the surface of the material web.
With DE 31 35 220 A1 a device for applying powder particles onto printed sheets of paper is disclosed in which the sheet is electrostatically charged on the side which is not to be powdered. This certainly has the advantage that the charging rods can be located relatively close to the sheet of paper, but the sheet of paper itself acts as an insulator. Locating the charging rod on the opposite side, i.e. the side of the sheet of paper to be powdered is possible, but the charging rod can only be located at a relatively long distance from the sheet, since space must be kept clear for the rapidly moving gripper.
The object of the invention, therefore, is to provide a device with which the printed sheet can be better powdered.
This object is met under the invention by locating the charging device outside the sheet delivery mechanism at a location before the sheet enters the sheet delivery mechanism.
This has the considerable advantage that the charging device can be positioned relatively close to the printed surface, because no clear space has to be created for any rapidly moving grippers. The grippers are located inside the sheet delivery mechanism, whereas the charging device is located outside the sheet delivery mechanism and, thus, the most suitable position can be selected. In addition, it has also been shown that electrostatic charging of the printed side of the sheet is more advantageous than charging the opposite side, i.e. the side which is not to be powdered.
Under the invention then, a part of the powdering process, which includes the electrostatic charging of the sheet of paper, is moved outside the sheet delivery mechanism. The application of the powder, however, continues to take place in the delivery mechanism. As a result of the immediate proximity of the charging device to the surface of the sheet to be powdered, small concentrated fields can be generated and act upon the sheet of paper, and because of the short distances, the field strength and thus the electrostatic charge can be adjusted precisely without any discharge resulting.
A further embodiment provides for the charging device to be designed as a charging rod. Firstly, this charging rod can be adjusted relatively easily to the paper quality, secondly, the distance of the charging rod to the adjacent surface of the web can be adjusted with great precision. In addition, the charging rod can be replaced without difficulty, for example, for repair and/or maintenance purposes.
Under a preferred example of the embodiment, the intended position of the charging device is in the area of the last pressure roller, viewed in the direction of transportation, of a rotary offset press. This has the considerable advantage that the web is held by the pressure roller and no gripper is needed either for holding or for transporting the sheet in this area, so that the charging device can be positioned relatively close to the sheet of paper. The charging device is immediately opposite the pressure roller. This means that the sheet of paper is located between the pressure roller and the charging device.
The best results are achieved when the intended position of the charging device is immediately ahead of the separation point from the pressure roller. In this case the sheet is released from the pressure roller immediately after it is electrostatically charged, so that no charge can drain off through the pressure roller.
In the example of one embodiment provision is made for positioning one or several ionization rods ahead of the charging rod. These ionization rods are used to discharge or to prepare the sheet, respectively, prior to charging by means of the charging rod. The ionization rods are supplied with alternating current (about 6.5 kV) and are used principally for actively discharging the sheet or the surface of the sheet. The charging rod on the other hand is operated with direct current (about 12 kV).
The pressure roller is preferably made from an electrically conductive material, for example, metal, and is grounded. The charging device, in particular the charging rod, is positively charged, or emits positive ions. This construction has the considerable advantage that one side of the sheet of paper can be grounded and, for example, the positive ions can be applied on the other side of the sheet of paper, and that negatively charged powder can be used.
It is preferable that there is no relative motion during charging between the sheet and the pressure roller, or ground. This allows an unhindered discharge of negative ions through ground. In addition, charging is not interrupted, because the sheet and ground are motionless relative to one another.
Then, for example, either electrostatically negatively charged powder or uncharged powder is applied to the electrostatically charged sheet. In the case of electrostatically charged powder, the powder has the opposite charge for charging the printed surface of the sheet of paper.
The powdering device can be positioned advantageously ahead of a sheet unroller. In general, the powdering device can be positioned ahead of the drying section. In any case, the powdering device is intended to be positioned ahead of a sheet braking device, so that when the sheet is braked it is already dusted with powder.
An additional advantage is also seen in the fact that the sheet is essentially being guided on cushions of air after it is separated from the pressure roller. In this way, any draining of the charge is prevented or at least delayed.
The charging device generates an electrostatic field matched to the width of the sheet, where the field can be adapted to the width of the sheet. In this way, when narrow sheets are being processed, damage to the pressure roller, for example, or to other pieces of machinery can be prevented.
Additional advantages, characteristics and details of the invention can be seen from the dependent claims and also from the following description, in which a particularly preferred embodiment is shown in detail, with reference to the drawing. In it the characteristics shown in the drawing and set forth in the claims and the description can be essential to the invention in each case individually or in any combination.